Mammals, birds, and reptiles constitute the amniote vertebrates. In all mammals and birds, and in many reptiles, sex is determined at fertilization by genotype (genotypic sex determination or GSD). In some reptiles, however, the temperature at which the egg incubates determines whether the embryo hatches as a male or a female (temperature-dependent sex determination or TSD) Thus, in these species, embryos are initially bipotential and only later does channelization toward the male or female phenotype occur. The initial bipotentiality and later determination of sex by species exhibiting TSD offer a unique model system for studies of the mechanisms of sex determination and sexual differentiation in all amniote vertebrates, including man. We have discovered recently that the administration of estrogen to embryos incubating at male-producing temperatures causes ovarian development, over-riding any temperature effect. The proposed research will investigate (i) how this estrogen effect occurs, (ii) whether, under normal conditions, steroid hormones are present before the gonad is committed to testicular/ovarian development, and (iii) whether embryonic steroids have a similar role in the differentiation of non-gonadal phenotypes as in mammals and birds. We also have discovered that the temperature that an embryo experiences profoundly affects its adult morphology, physiology, and behavior. The mechanisms by which incubation temperature organizes adult sexuality will be studied through hormonal manipulations of embryos and neonates by castration and/or administration of steroid hormones, synthetic hormone agonists and antagonists, enzyme inhibitors, and steroid antisera. Together, these experiments will determine the extent to which GSD and TSD have similar physiological or biochemical bases in the control of gonad determination and in the biopsychology of sexual differentiation. The proposed studies are of fundamental importance to understanding, the development of sexuality in vertebrates, including the relation of different levels of sexuality within an individual. The studies will use multiple techniques, including behavioral testing, histology, -immunocytochemistry, monoclonal antibodies for hormone receptors, synthetic steroid agonists and antagonists, high-performance liquid chromatography, gas chromatography/mass. spectrometry, and radioimmunoassay.